Temperature-independent Point Research Articles

Localization and interference induced quantum effects at low magnetic fields in InGaAs/GaAs structures

The longitudinal ρxx(B, T) and Hall ρxy(B, T) resistances are experimentally investigated in n-InGaAs/GaAs nanostructures with a single and double quantum wells in the magnetic field range B = 0–2.5 T and temperatures T = 1.8–20 K. It is shown that the origin of the temperature-independent point located at ωcτ≅1 on the ρxx(B, T) curves is due to the combined action of the classical cyclotron motion and the quantum interference effects of weak localization and electron-electron interaction. The results obtained indicate that the transition from the dielectric phase to the phase of the quantum Hall effect is a crossover from weak localization (quantum interference effects in a weak magnetic field) to strong localization in quantizing magnetic fields in the quantum Hall effect regime.

Tradeoff between the transistor reconfigurable technology and the zero-temperature-coefficient (ZTC) bias point on BESOI MOSFET

A tradeoff between transistor reconfigurability and a Zero Temperature Coefficient (ZTC) bias point was observed and analyzed for Back Enhanced SOI (BESOI) MOSFET for the first time. The ZTC is observed only for BESOI MOS with aluminum Source/Drain (S/D) electrodes (ohmic contact with the channel) while the transistor reconfigurability is present only for BESOI MOS with nickel S/D electrodes (Schottky contact). This tradeoff is discussed and demonstrated experimentally and by numerical simulations.For ohmic S/D contacts, the temperature-independent point (ZTC) happens for front gate bias from −2.44 V to −0.61 V (for a back gate VGB changing from −15 V to −30 V respectively). Following other technologies, such as UTBB and FinFET, this point represents the compensation between the threshold voltage decrease and the mobility degradation when increasing the temperature. However, for S/D to channel Schottky contacts, there is no ZTC point and only for this condition the BESOI presents a reconfigurable behavior, i.e., p-type and n-type BESOI behavior can be observed by applying different VGB.The BESOI CMOS inverter built with nickel S/D electrodes was used to confirm the transistor reconfigurability. Therefore, depending on the S/D electrodes material, a reconfigurable device or a ZTC bias point behavior on BESOI MOSFET is observed, which is useful for different applications at the same chip.

Quantum magnetotransport inn-InGaAs/GaAs structures with electron density changes caused by infrared radiation

An experimental study of the longitudinal ρxx(B, T) and Hall ρxy (B, T) magnetoresistance as a function of the n-InGaAs/GaAs nanostructure transverse magnetic field, with single or double tightly-coupled quantum wells, depending on the width of the well, for magnetic field B = 0–12 T and temperature T = 0.05–100 K ranges, before and after low-temperature illumination by infrared radiation. Before illumination, a change in the samples' temperature dependence of zero-field resistivity ρ(T) was detected, from “dielectric” (dρ/dT < 0) to “metallic” (dρ/dT > 0). It is shown that the temperature dependence of resistivity is set by the mobility temperature dependence μ(T), the “dielectric” portion of which is related to the quantum corrections to conductivity in the diffusion and ballistic regimes, whereas the “metallic” portion is associated to the scattering of the carriers by acoustic and optic phonons. A slight change in the magnetic-field dependence of the longitudinal magnetoresistance ρxx(B, T) was observed with temperature, near the induction value corresponding to μB = 1. We also found unusual temperature dependence for conductivity components σxx(B, T) and σxy (B, T), at μB = 1. The σxx(B, T) curve has an observable temperature-independent point, whereas σxy (B, T) strongly depends on T at μB = 1. It is established that this regularity is caused by the nature of the temperature dependence of the charge carrier mobility μ(T), in both the diffusion and ballistic regimes. After IR illumination, positive persistent photoconductivity is observed in all samples, associated with a twofold increase in the charge carrier concentration. Resistivity in a zero-magnetic field ρ(T) for such samples also transitions from “dielectric” to “metallic” conductivity, at temperatures lower than before illumination. It is shown that the particularities of the transport after illumination are related to the manifestation of charge carrier concentration temperature dependence.

Dirac fermion heating, current scaling, and direct insulator-quantum Hall transition in multilayer epitaxial graphene

We have performed magnetotransport measurements on multilayer epitaxial graphene. By increasing the driving current I through our graphene devices while keeping the bath temperature fixed, we are able to study Dirac fermion heating and current scaling in such devices. Using zero-field resistivity as a self thermometer, we are able to determine the effective Dirac fermion temperature (TDF) at various driving currents. At zero field, it is found that TDF ∝ I≈1/2. Such results are consistent with electron heating in conventional two-dimensional systems in the plateau-plateau transition regime. With increasing magnetic field B, we observe an I-independent point in the measured longitudinal resistivity ρxx which is equivalent to the direct insulator-quantum Hall (I-QH) transition characterized by a temperature-independent point in ρxx. Together with recent experimental evidence for direct I-QH transition, our new data suggest that such a transition is a universal effect in graphene, albeit further studies are required to obtain a thorough understanding of such an effect.

Tunable insulator-quantum Hall transition in a weakly interacting two-dimensional electron system.

We have performed low-temperature measurements on a gated two-dimensional electron system in which electron–electron (e-e) interactions are insignificant. At low magnetic fields, disorder-driven movement of the crossing of longitudinal and Hall resistivities (ρxx and ρxy) can be observed. Interestingly, by applying different gate voltages, we demonstrate that such a crossing at ρxx ~ ρxy can occur at a magnetic field higher, lower, or equal to the temperature-independent point in ρxx which corresponds to the direct insulator-quantum Hall transition. We explicitly show that ρxx ~ ρxy occurs at the inverse of the classical Drude mobility 1/μD rather than the crossing field corresponding to the insulator-quantum Hall transition. Moreover, we show that the background magnetoresistance can affect the transport properties of our device significantly. Thus, we suggest that great care must be taken when calculating the renormalized mobility caused by e-e interactions.

Experimental evidence for direct insulator-quantum Hall transition in multi-layer graphene

We have performed magnetotransport measurements on a multi-layer graphene flake. At the crossing magnetic field Bc, an approximately temperature-independent point in the measured longitudinal resistivity ρxx, which is ascribed to the direct insulator-quantum Hall (I-QH) transition, is observed. By analyzing the amplitudes of the magnetoresistivity oscillations, we are able to measure the quantum mobility μq of our device. It is found that at the direct I-QH transition, μqBc ≈ 0.37 which is considerably smaller than 1. In contrast, at Bc, ρxx is close to the Hall resistivity ρxy, i.e., the classical mobility μBc is ≈ 1. Therefore, our results suggest that different mobilities need to be introduced for the direct I-QH transition observed in multi-layered graphene. Combined with existing experimental results obtained in various material systems, our data obtained on graphene suggest that the direct I-QH transition is a universal effect in 2D.

Selection of gate length and gate bias to make nanoscale metal–oxide-semiconductor transistors less sensitive to both statistical gate length variation and temperature variation

Aggressive scaling of transistors leads to an ever-increasing amount of process variations. In this work, we studied the gate length dependency of on-current ( I on), off-current ( I off), effective drive current ( I eff), saturation threshold voltage ( V th,sat), and temperature independent point (TIP). Experimental evidence show that the gate length dependency of TIP in nanoscale transistors is related to the V th,sat versus L characteristics rather than velocity saturation. We found that I on, I off and I eff of nanoscale transistors in the transition between reverse short channel effect (RSCE) and short channel effect (SCE) are less sensitive to gate length variation and temperature variation.

From insulator to quantum Hall liquid at low magnetic fields

We have performed low-temperature transport measurements on a GaAs two-dimensional electron system at low magnetic fields. Multiple temperature-independent points and accompanying oscillations are observed in the longitudinal resistivity between the low-field insulator and the quantum Hall (QH) liquid. Our results support the existence of an intermediate regime, where the amplitudes of magneto-oscillations can be well described by conventional Shubnikov--de Haas theory, between the low-field insulator and QH liquid.

Temperature stability of a dichroic atomic vapor laser lock

We have investigated the temperature stability of the dichroic atomic vapor laser lock laser frequency lock method. We find that, in general, the lock exhibits significant temperature sensitivity, leading to laser frequency drifts as large as tens of MHz/K. However, for certain configurations of the optical elements of the system, this temperature dependence is reduced to below 1 MHz/K. These temperature-independent points can be found across a broad range of frequencies. We present a numerical model that reproduces the general behavior of the system.

Magnetic-field-induced phase transitions in a Si/SiGe hole system

We have performed low-temperature transport experiments on a Si/SiGe hole system. The measured transverse and longitudinal conductivities σ xy and σ xx allow us to study the magnetic-field-induced transitions in this system. In particular, we present the study of temperature-driven flow lines in the “anomalous Hall insulator” regime near a Landau level filling factor ν=1.5. The “anomalous” temperature-driven flow lines could be due to the unusual energy level scheme in a Si/SiGe hole system. Moreover, for 3< ν<5, there is a temperature-independent point in ρ xx ( B), ρ xy ( B), σ xx ( B), and σ xy ( B) which corresponds to a boundary of the quantum phase transition.

Insulator-quantum Hall transitions in two-dimensional electron gas containing self-assembled InAs dots

We present a study of magnetotransport properties in two-dimensional electron gases formed in a GaAs/Al 0.33Ga 0.67As quantum well, where self-assembled InAs quantum dots have been inserted into the centre of the GaAs well. In a perpendicular magnetic field there are transitions between quantum Hall liquids at filling factors ν=2 and 1 and the insulating phase from temperature independent points. In the insulating regime we observe ν=4 and 6 quantum Hall states due to Landau quantisation. We are able to construct a phase diagram in which Shubnikov–de Haas oscillations and insulating behaviour coexist.

Insulator–quantum Hall liquid transitions in a two-dimensional electron gas using self-assembled InAs dots

We investigate the transport properties of two-dimensional electron gases (2DEG) formed in a GaAs/AlGaAs quantum well, where self-assembled InAs quantum dots were grown at the center of the GaAs well. Due to the resulting strain fields repulsive short-range scattering is experienced by the conduction electrons in the 2DEG. In a perpendicular magnetic field, there are transitions between quantum Hall liquids at filling factors ν=1 and 2 and the insulating phase. We show that the boundary of insulator–Quantum Hall transitions can be identified either by analysing the temperature-independent points in ρ xx or from the peaks in σ xx at low temperatures and both methods give similar results.

Quantum magneto-transport in two-dimensional GaAs electron gases and SiGe hole gases

We have measured the low-temperature transport properties of two-dimensional (2D) GaAs electron gases and 2D SiGe hole gases. Our experimental results fall into three categories. (i) Collapse of spin-splitting and an enhanced Landé g-factor at Landau level filling factors both ν=3 and ν=1 in a 2D GaAs electron gas are observed. Our experimental results show direct evidence that the effective disorder is stronger at ν=1 than that at ν=3 over approximately the same perpendicular magnetic field range. (ii) We present evidence for spin-polarisation of a dilute 2D GaAs electron gas. The Lande g-factor of the system is estimated to be 1.66. This enhanced g value is ascribed to electron–electron interactions at ultra low carrier density limit. (iii) In a high-quality SiGe hole gas, there is a temperature-independent point in the magnetoresistivity ρ xx and ρ xy which is ascribed to experimental evidence for a quantum phase transition between ν=3 and ν=5. We also present a study on the temperature( T)-driven flow lines in our system.

On-chip p-MOSFET dosimetry (CMOS ICs)

On-chip p-FETs were developed to monitor the radiation dose of n-well CMOS ICs by monitoring threshold voltage shifts due to radiation-induced oxide and interface charge. The design employs closed-geometry FETs and a zero-biased n-well to eliminate leakage currents. The FETs are operated using a constant current chosen to greatly reduce the FET's temperature sensitivity. The dose sensitivity of these p-FETs is about -2.6 mV/krad(Si) and the off-chip instrumentation resolves about 440 rad(Si)/b. When operated with a current at the temperature-independent point, it was discovered that the preirradiation output voltage is about -1.5 V, which depends only on design-independent silicon material parameters. The temperature sensitivity is less than 63 mu V/ degrees C over a 70 degrees C temperature range centered about the temperature-insensitive point.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>